Reference is made to FIG. 1 which schematically illustrates a conventional capacitive touch screen system 10. The touch screen system 10 includes a touch screen panel 10p that is formed to include an array of capacitive sense nodes 12. Each sense node 12 is located at a point where a drive (or force) line 14 crosses a sense line 16. In a common implementation, the drive and sense lines 14 and 16 are formed of a conductive material. The plurality of drive lines 14 are formed in a first material layer that is supported by a substrate. The plurality of sense lines 16 are formed in a second material layer that is also supported by the substrate. The first and second material layers are separated by a layer of dielectric material. At each sense node 12 a capacitor 12c is thus formed which is comprised of a first plate 14a (from the portion of the drive line 14 at the point of crossing), a second plate 16a (from the portion of the sense line 16 at the point of crossing) and a dielectric 18 positioned between the first and second plates. The capacitor 12c at each sense node 12 will exhibit an associated mutual (or coupling) capacitance value as known in the art.
Although the drive and sense lines 14 and 16 are illustrated as linear structures, it will be understood that this is merely one typical implementation and that it is known in the art to form drive and sense lines having shapes other than linear. For example, serially connected diamond shapes are well known in the art to form drive/sense lines.
Furthermore, although the drive and sense lines 14 and 16 are described as being located in different layers, it will be understood that this is merely one typical implementation and that it is known in the art to form the drive and sense lines in a common material layer, with the point of crossing at sense nodes 12 being provided through a conductive bridge and intervening dielectric structure.
The drive lines 14 are activated by force signals output by drive circuits 20 coupled to the drive lines. In a common implementation, the force signals output from the drive circuits 20 may comprise an AC signal. The sense lines 16 are coupled to the inputs of sense circuits 22 which may operate as sense amplifiers (for example, charge amplifiers or transconductance amplifiers) to generate an output signal.
Because of the application of an AC signal to the drive lines, and the presence of a capacitive coupling to the sense lines, the output signal generated by each sense circuit 22 will be indicative of the capacitance at the sense node 12. For example, the AC signal applied by a drive circuit 20 to a given drive line 14 is coupled through the capacitor 12c at a sense node 12 to the crossing sense line 16. The sense circuit 22 coupled to the crossing sense line 16 receives the coupled AC signal and detects a voltage on the sense line. The magnitude of the sensed voltage varies as a function of the mutual (or coupling) capacitance for the capacitor 12c at the sense node 12.
The presence of an object, such as a human body part (for example, a finger) or device (for example, a stylus) near the sense node 12 causes a change in the mutual (or coupling) capacitance for the capacitor 12c at that sense node 12. As a result, there will be a change in the coupled AC signal, and a corresponding change in the magnitude of the voltage sensed by the sense circuit 22, as a result of the object's presence and its effect on the mutual (or coupling) capacitance for the capacitor 12c at the sense node 12.
A control circuit 26 is coupled to the drive circuits 20 and sense circuits 22. The control circuit 26 includes a drive controller 28 which operates to sequentially actuate each of the drive circuits 20 to apply the AC signal to each drive line 14. The control circuit 26 further includes a signal processing circuit 30 coupled to the outputs of the sense circuits 22. For each drive controller 28 actuation of a drive circuit 20, the signal processing circuit 30 operates to read the voltage of the coupled AC signal as sensed by each of the sense circuits 22. As a result, a voltage value is collected by the signal processing circuit 30 from each sense node 12 of the capacitive touch screen 10. The collected voltage values are then processed by the signal processing circuit 30 to determine presence of the object and the location (or locations) of that object.
In this context, detecting the presence of the object may include in some configurations the ability of the control circuit 26 to detect from the collected voltage values both an actual touch of the capacitive touch screen by the object as well as instances where the object is close to, but not touching, the capacitive touch screen (this being referred to in the art as a “hover” detection).
The detection by the signal processing circuit 30 of object presence is typically made by comparing the collected voltage values at the sense nodes 12 to a threshold value. If a sufficient number of voltage values in an area of the capacitive touch screen 10 are determined by the signal processing circuit 30 to exceed the threshold value, the control circuit 26 will generate an output signal indicative of the detection and location of object presence at that area.
Reference is now made to FIG. 2A which illustrates an overlay of sensed voltage values at the sense nodes 12 for a portion of the capacitive touch screen 10. In this case, the voltage values, here represented by a corresponding signal code value generated by the signal processing circuit 30 in response to the sense circuit 22 output, were sensed as a result of an actual touch 40 of the capacitive touch screen by a human finger. The magnitude of the signal code values is higher at sense nodes closer to the point where finger contact is made with the capacitive touch screen. It will be noted that a large number of sensed values have magnitudes in excess of an exemplary threshold value of “200”. This will be interpreted by the signal processing circuit 30 of the control circuit 26 as a detected touch of the capacitive touch screen 10 and a corresponding output signal is generated. For a finger touch, in an exemplary implementation, the signal code values associated with a touch may even be as high as 800-1000, and thus the illustrated values in FIG. 2A are for example only.
Reference is now made to FIG. 2B which again illustrates an overlay of sensed voltage values at the sense nodes 12 for a portion of the capacitive touch screen 10. In this case, the voltage values (represented by corresponding signal code values) were sensed as a result of an actual touch 42 of the capacitive touch screen by a stylus. The magnitude of the signal code values is higher at sense nodes closer to the point where stylus contact is made with the capacitive touch screen. However, because the touch area for a stylus is much smaller than the touch area for a human finger, there are fewer sense nodes which have signal code values. If the exemplary threshold of “200” from FIG. 2A is used in this case, the signal processing circuit 30 of the control circuit 26 will not indicate the detection of a touch because an insufficient number of sense nodes 12 have signal code values in excess of the exemplary threshold. For a stylus touch, in an exemplary implementation, the signal code values associated with a touch are typically less than those associated with a finger touch (for example, less than 800), and thus the illustrated values in FIG. 2B are for example only.
To ensure that the stylus touch 42 is in fact detected, one option is to lower the threshold value. For example, if the threshold value is lowered to a threshold value of “100”, for example, then a sufficient number of sense nodes 12 (in this case two or mode) will have signal code values in excess of the lower threshold. This will be interpreted by the signal processing circuit 30 of the control circuit 26 as a detected touch of the capacitive touch screen 10. It will be understood that a stylus detection can be made from a single node 12 value in excess of the threshold.
The foregoing analysis is made without consideration of the time domain. If the time domain is considered in the calculation, the lowering of the threshold value so as to permit making the stylus touch detection can produce a problem with respect to the accuracy of the touch detection operation.
Consider now the situation where the human finger is moving towards the capacitive touch screen, but has not yet make an actual touch contact with the capacitive touch screen. FIG. 2C illustrates an overlay of sensed voltage values at the sense nodes 12 for a portion of the capacitive touch screen 10 at a point in time in advance of the actual touch event occurrence. The voltage values, here again represented by corresponding signal code values, are in effect sensing a “hover” by the moving human finger over the capacitive touch screen. The magnitude of the signal code values is higher at sense nodes closer to the point where the finger hovers over the capacitive touch screen. However, it will be noted that all the signal code values have a much lower magnitude in comparison to the instance of an actual touch as described above in connection with FIG. 2A. If the exemplary threshold value of “200” for a finger touch detection (FIG. 2A) is applied in this case, no (or an insufficient number of) signal code values will exceed the exemplary threshold and the signal processing circuit 30 of the control circuit 26 will correctly identify that no touch of the capacitive touch screen has (as yet) occurred.
However, if the lowered threshold value of “100” (as discussed above in connection with FIG. 2B) is instead applied, in a manner consistent with wanting to permit the making of a stylus touch detection, then it will be noted that a sufficient number of sense nodes 12 (in this case two or more) have signal code values in excess of that lower threshold. So, the hover condition of the moving finger will then be incorrectly interpreted by the signal processing circuit 30 of the control circuit 26 as an actual detected (stylus) touch of the capacitive touch screen 10.
Importantly, the incorrect detection of the hover condition as an actual touch of the capacitive touch screen is premature in the time domain with respect to the movement of the finger towards the capacitive touch screen. Those skilled in the art will recognize that such premature (incorrect) touch detection can cause significant problems for the execution of applications (such as games) which rely on an accurately timed detection of a touch as a control input. This can be a significant source of user dissatisfaction with the operation of the capacitive touch screen.
There is accordingly a need in the art for a way to more accurately make different types of detections in connection with the operation of a capacitive touch screen.